Methods involving deposition of a metal oxide layer via liquid phase decomposition (hydrolysis) of a corresponding salt (i.e. sulfate or halide) are known per se and have been used to form luster, or pearlescent pigments which have translucent, non-reflective mica core materials. However, such methods, described for example in U.S. Pat. Nos. 3,087,827 and 5,733,371, incorporated herein in their entirety by reference, have not been considered suitable for forming effect pigments with reflective metallic cores in the highly acid (pH of less than 4), aqueous solutions required by such processes. U.S. Pat. No. 6,369,147, incorporated herein in its entirety by reference, discloses a process that solves the foregoing problem by selecting certain metal cores and optionally treating them in such a way that they are rendered more corrosion resistant.
Use of microwave energy for the deposition of metal oxide films onto glass and indium tin oxide coated glass plates used for LED devices is known and disclosed in numerous journal articles such as E. Vigil, L. Saadoun, Thin Solid Films 2000, 365, pp 12-18 and E. Vigil, L. Saadoun, J. Materials Science Letters 1999, 18 pp 1067-1069. Good adhesion was obtained only on indium tin oxide coated glass plates, which the authors suggested was due to some electron donation ability of the indium tin oxide coating (see Vigil, E.; Ayllón, J. A.; Peiró, A. M.; Rodriguez-Clemente, R.; Domènech, X.; Peral, J. Langmuir2001, 17, 891).
The bulk precipitation of metal oxide particles by microwave irradiation is well known. For examples of bulk precipitation oxides using microwave deposition, see (1) Lerner, E.; Sarig, S.; Azoury, R., Journal of Materials Science: Materials in Medicine 1991, 2, 138 (2) Daichuan, D.; Pinjie, H.; Shushan, D. Materials Research Bulletin, 1995, 30, 537 (3) Leonelli, C. et al., Microwaves: Theory and Applications in Materials Processing 2001, 111, 321, (4) Girnus, I. et al., Zeolites 1995, 15, 33, (5) Rodriguez-Clemente, R. et al., Journal of Crystal Growth 1996, 169, 339 and (6) Daichuan, D.; Pinjie, H.; Shushan, D. Materials Research Bulletin, 1995, 30, 531.
The present invention provides a method whereby precursors of one or more desired metal oxides are converted into a metal oxide layer on a substrate using microwave radiation to generate, after separating the metal oxide layer from the substrate, plane-parallel structures, e.g., flakes, composed of the desired metal oxide or metal oxides. The plane-parallel structures are formed as thin particles, approximately 500 nm thick or less. Flakes can be produced having a smooth mirror-like surface and a high aspect ratio and can be used as substrate material for effect pigments.
Porous structures can also be produced via the present method by incorporating soluble polymers, particularly water soluble polymers, into a composition comprising the metal oxide precursors.
The plane-parallel structures are useful in many applications including interference pigments and effect pigments. The porous structures provide excellent host cites for a variety of materials such as catalysts, colorants, anti-microbial compounds etc.